Core Concept

Decomposition reactions involve a single compound breaking down into two or more simpler substances. These reactions follow the general form:

AB→A+B\text{AB} \rightarrow \text{A} + \text{B}AB→A+B

Where:

• AB is a single compound (the reactant).

• A and B are the products, which can be elements or simpler compounds.

Decomposition reactions are often driven by energy input, such as heat, light, or electricity.

Types of Decomposition Reactions

1. Thermal Decomposition:

   • The compound decomposes when heated.

   • Example: CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3​→CaO+CO2​ (heating calcium carbonate produces calcium oxide and carbon dioxide).

2. Electrolytic Decomposition:

   • The compound decomposes when an electric current is passed through it.

   • Example: 2H2O→2H2+O2\text{2H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_22H2​O→2H2​+O2​ (electrolysis of water produces hydrogen and oxygen gases).

3. Photodecomposition (Photolysis):

   • The compound decomposes when exposed to light.

   • Example: 2AgCl→2Ag+Cl2\text{2AgCl} \rightarrow 2\text{Ag} + \text{Cl}_22AgCl→2Ag+Cl2​ (sunlight decomposes silver chloride into silver and chlorine gas).

4. Catalytic Decomposition:

   • A catalyst speeds up the decomposition without being consumed.

   • Example: 2H2O2→2H2O+O2\text{2H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_22H2​O2​→2H2​O+O2​ (decomposition of hydrogen peroxide is often catalyzed by manganese dioxide).

Recognizing Decomposition Reactions

To identify a decomposition reaction:

• Look for a single reactant that produces multiple products.

• Common indicators include heat, light, electricity, or a catalyst, which are often required to drive the reaction.

Predicting Products in Decomposition Reactions

1. Binary Compounds:

   • Compounds made of two elements decompose into their individual elements.

   • Example: HgO→Hg+O2\text{HgO} \rightarrow \text{Hg} + \text{O}_2HgO→Hg+O2​

2. Metal Carbonates:

   • Metal carbonates decompose upon heating to produce a metal oxide and carbon dioxide.

   • Example: CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3​→CaO+CO2​

3. Metal Hydroxides:

   • Metal hydroxides decompose upon heating to produce a metal oxide and water.

   • Example: Ca(OH)2→CaO+H2O\text{Ca(OH)}_2 \rightarrow \text{CaO} + \text{H}_2\text{O}Ca(OH)2​→CaO+H2​O

4. Metal Chlorates:

   • Metal chlorates decompose upon heating to produce a metal chloride and oxygen gas.

   • Example: 2KClO3→2KCl+3O2\text{2KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_22KClO3​→2KCl+3O2​

5. Oxyacids:

   • Oxyacids (acids containing oxygen) decompose to form nonmetal oxides and water.

   • Example: H2CO3→CO2+H2O\text{H}_2\text{CO}_3 \rightarrow \text{CO}_2 + \text{H}_2\text{O}H2​CO3​→CO2​+H2​O